The present invention relates to methods for coating metallic articles having oxides thereon.
Nitinol, a class of nickel-titanium alloys, is well-known for its shape memory and pseudoelastic properties, making it amenable to a wide range of applications.
As a shape memory material, nitinol is able to undergo a reversible thermoelastic transformation between certain metallurgical phases. Generally, the thermoelastic shape memory effect allows the alloy to be shaped into a first configuration while in the relative high-temperature austenite phase, cooled below a transition temperature or temperature range at which the austenite transforms to the relative low-temperature martensite phase, deformed while in a martensitic state into a second configuration, and heated back to austenite such that the alloy transforms from the second configuration to the first configuration.
As a pseudoelastic material, nitinol is able to undergo an isothermal, reversible transformation from austenite to martensite upon the application of stress. The elasticity associated with the transformation to martensite and the resulting stress-induced martensite make pseudoelastic nitinol suitable for applications requiring recoverable, isothermal deformation. For example, conventional pseudoelastic nitinol is useful for applications requiring recoverable strains of up to 8% or more.
Since being discovered by William J. Buehler in 1958, the unique properties of nitinol have been applied to numerous applications. For example, as reported in C. M. Wayman, xe2x80x9cSome Applications of Shape-Memory Alloys,xe2x80x9d J. Metals 129 (June 1980), incorporated herein by reference, nitinol has been used for applications such as fasteners, couplings, heat engines, and various dental and medical devices. Owing to the unique mechanical properties of nitinol and its biocompatibility, the number of uses for this material in the medical field has increased dramatically in recent years.
Implantable medical devices such as stents, blood filters, hemostatic clips, prostheses, and the like are often made from nitinol. Because of the elastic properties and shape memory characteristics of nitinol, these medical devices are capable of being compressed to a reduced configuration for insertion into the body and then expanded by self-expansion or mechanical expansion once positioned to a target location within the body. The position of these devices while moved within the body is often observed by fluoroscopic techniques, during which the device is visualized by x-radiation. As such, it is desired that the device be highly radiopaque. Nitinol, however, is not a highly radiopaque material.
One method of increasing the radiopacity of nitinol medical devices is to apply a coating of a highly radiopaque material to the external surface of the device by a process such as electroplating. However, effective electroplating is often difficult to achieve because of contaminants on the nitinol external surface, which result in potential problems such as poor adhesion of the plated coating. One such contaminant is oxide that readily forms on nitinol when exposed to an oxygen-containing atmosphere. Conventional methods may not be effective in removing such oxides and ensuring that such nitinol surfaces remain substantially oxide-free prior to the coating process. Moreover, conventional coating techniques often necessitate the use of hazardous chemicals. There thus exists a need for safe methods of removing oxide and other contaminants from nitinol-surfaces and keeping such surfaces clean and substantially oxide-free prior to the coating process.
In one aspect, the present invention relates to methods for coating metallic articles that have oxides thereon. In an exemplary embodiment, the method comprises the steps of providing a metallic article having an external surface with an oxide thereon; removing at least part of the oxide from the external surface of the article; and placing a coating on the article. In another aspect, the present invention relates to coated metallic articles. In yet another aspect, the invention relates to an apparatus for coating metallic articles.
One advantage of the present invention is that it provides methods for removing oxide from metallic surfaces so that the adhesion and integrity of subsequently applied coatings is enhanced.
Another advantage of the present invention is that it provides methods for removing oxide from metallic surfaces and applying radiopaque coatings without the use of hazardous chemicals.
Yet another advantage of the present invention is that it provides methods of removing oxides from metallic surfaces without creating a rough surface or causing the removal of excessive material, thus making it possible to treat delicate articles such as medical devices.